This application claims convention priority based on Japanese Patent Applications No. 2001-63168 filed on Mar. 7, 2001, and 2001-295743 filed on Sep. 27, 2001. These Japanese patent Applications are incorporated by reference in this application.
1. Field of the Invention
The present invention relates to a small antenna element suitable for use in a mobile telecommunication device, in particular, to a surface-mounted antenna element.
2. Description of the Related Art
An antenna element used in a mobile telecommunication device may often be a linear antenna element, in particular, a half-wave antenna element having a length one-half a wavelength for a used frequency to produce resonance. However, for miniaturization of antennas, a monopole antenna consisting of a quarter-wave radiation electrode has come into use.
While the quarter-wave monopole antenna can be miniaturized easier than the half-wave antenna because of its shorter radiation electrode, it has a problem in that a radiation characteristic thereof is disturbed by an induced current occurring in a board-grounding conductor or housing for electromagnetically shielding a circuit of the telecommunication device. To solve this problem, in U.S. Pat. No. 5,517,676 issued May 14, 1996 and U.S. Pat. No. 5,903,822 issued May 11, 1999, there has been proposed a technique of using a quarter-wave monopole antenna and canceling the effect of the induced current flowing through a housing by forming a recess in the housing at a position distant from an antenna feeding point by a quarter of a wavelength for a used frequency. Besides, a technique of canceling the effect of the induced current by providing a stub having a length of a quarter of the wavelength has been proposed. However, these techniques contradict miniaturization. On the contrary, the half-wave antenna element has the advantage of being less affected by the board-grounding surface. However, since the half-wave antenna requires the radiation electrode longer than that of the quarter-wave antenna, it is not suitable for miniaturization, and therefore has typically been used as the monopole antenna pulled out of the telecommunication device.
Furthermore, a chip antenna, which is a small chip, having a radiation electrode formed on a dielectric substrate has the advantage that the antenna element can be miniaturized and the substrate can be mounted on a printed wiring board. However, it has the disadvantage that an available frequency bandwidth is narrow.
Thus, an object of the present invention is to provide a small antenna element with a stable characteristic that can be enhanced in radiation efficiency and bandwidth thereof.
Another object of the present invention is to provide a telecommunication device having the antenna element mounted thereon, for example, a telecommunication device mounted on a cellular phone, a headphone, a personal computer, a notebook PC, a digital camera or the like as an antenna for Bluetooth.
Another object of the present invention is to provide an antenna element having a radiation electrode of a shape symmetric with respect to the center thereof, both the halves of the radiation electrode being matched in impedance, and capable of producing enhanced resonance in the antenna portion, and a telecommunication device having the antenna element.
An antenna element according to the present invention comprises a dielectric substrate, and a radiation electrode of an electric conductor formed mainly on a surface of the dielectric substrate. The dielectric substrate is a dielectric chip, preferably a hexahedron of dielectric material. The antenna element has a power supply conductor and a ground conductor, which are connected to the radiation electrode, on the dielectric substrate, preferably on a surface other than the surface of the dielectric substrate on which the radiation electrode is formed. The radiation electrode has first and second halves, the first and the second halves being substantially symmetric in form to one another with respect to the center of the radiation electrode and being to radiate with the same direction of main polarization of radiation emitted from the radiation electrode. The first half has a first open end at its outer end and a first connection terminal adjacent to the center. The second half has a second open end at its outer end and a second connection terminal adjacent to the center, the second connection terminal being at a distance from the first connection terminal on the radiation electrode. A power supply conductor is formed on the dielectric substrate and connected to the first connection terminal at one end thereof and has at the other end a terminal for connecting to a high frequency signal source. A ground conductor is formed on the dielectric substrate and connected to the second connection terminal at one end thereof and has at the other end a terminal for connecting to a ground.
A portion of the first half between the first open end and the first connection terminal is asymmetric in form to a portion of the second half between the second open end and the second connection terminal. Alternatively, the power supply conductor is asymmetric in form to the ground conductor. Due to this asymmetric form, the total impedance of the power supply conductor and the portion of the first half between the first open end of the first half and the terminal of the power supply conductor at the other end for connecting to a high frequency signal source and the internal impedance of the high frequency signal source can substantially match, in total impedance, the ground conductor and the portion of the second half between the second open end of the second half and the terminal of the ground conductor at the other end for connecting to a ground.
In the antenna element according to this invention, it is preferred that the first and the second halves of the radiation electrode connect capacitively to a ground at the first and at the second open ends, respectively. Further preferably, the antenna element further comprises ground electrodes, formed adjacent to the first and the second open ends on the dielectric substrate, for connecting a ground, each of the ground electrodes connecting capacitively to the first and the second halves of the radiation electrode at the first and at the second open ends, respectively.
The radiation electrode of the antenna element according to this invention is preferably in a meandering form. Since the meandering form allows the radiation electrode to be mounted on a small surface of the dielectric substrate even if the radiation electrode is long, the size of the antenna element can be reduced.
The electric conductor forming the radiation electrode may be discontinuous between the first connection terminal and the second connection terminal and divided into the first and the second halves. Alternatively, the electric conductor forming the radiation electrode may be continuous from the first half to the second half and have one of the first and the second connection terminals around the center of the radiation electrode.
Each of the first and the second halves may be a quarter-wave antenna. Here, the xe2x80x9cquarter-wave antennaxe2x80x9d refers to a radiation electrode that has an electrical equivalent length of a quarter of a wavelength for a used frequency to produce resonance.
In the antenna element according to this invention, the electric conductor width of each of the first and the second halves of the radiation electrode may be narrowing from the center toward each of the open ends and the distance between the electric conductors of each of the first and the second halves may be increasing from the center toward each of the open ends.
According to this invention, on a surface of the dielectric substrate on which the radiation electrode is formed, another dielectric substrate may be provided to bury the radiation electrode in the dielectric. The length of the dipole radiation electrode, which is needed to produce resonance at the wavelength related with the frequency of the radiation used by the mobile telecommunication device, depends on an effective dielectric constant xcex5reff of the substrate having the radiation electrode thereon. Specifically, the length is represented by xcex/4xc3x971/xcex5reff for the quarter-wave antenna, indicating that the length is in inverse proportion to xcex5reff. Preferred materials for the dielectric substrate are glass fabric based epoxy resin and alumina ceramics having an effective dielectric constant of about 4 and about 8 to 10, respectively. The higher the effective dielectric constant of the substrate, the shorter the radiation electrode can be made, and burying the radiation electrode in the dielectric can assure the advantage of using the dielectric.
While in the above description, the radiation electrode made of a conductor is formed mainly on one surface of the dielectric substrate, the whole radiation electrode made of a conductor may be formed on that one surface of the dielectric substrate. Alternatively, in the antenna element of this invention, most part of the radiation electrode may be formed on one side of the substrate, and the remainder of the radiation electrode may be formed on a side adjacent to that side.
A telecommunication device according to this invention comprises a printed wiring board and an antenna element mounted on the printed wiring board. The printed wiring board has a ground area of the board with a ground conductor, a ground-free area of the board without a ground conductor and a high frequency signal lead. The antenna element comprises a dielectric substrate, and a radiation electrode of an electric conductor formed mainly on a surface of the dielectric substrate. The dielectric substrate is a dielectric chip, preferably a hexahedron of dielectric material. The antenna element has a power supply conductor and a ground conductor, which are connected to the radiation electrode, on the dielectric substrate, preferably on a surface other than the surface of the dielectric substrate on which the radiation electrode is formed. The antenna element is mounted on the ground-free area of the board so that a dielectric substrate surface other than the dielectric substrate surface on which the radiation electrode is formed faces on the ground-free area.
The radiation electrode having a first and a second halves, the first and the second halves being substantially symmetric in form to one another with respect to the center of the radiation electrode and being to radiate with the same direction of main polarization of radiation emitted from the radiation electrode. The first half has a first open end at its outer end and a first connection terminal adjacent to the center. The second half has a second open end at its outer end and a second connection terminal adjacent to the center, the second connection terminal being at a distance from the first connection terminal on the radiation electrode. A power supply conductor is formed on the dielectric substrate and connected to the first connection terminal at one end of the power supply conductor and has at the other end a terminal connected to the high frequency signal lead on the printed wiring board. A ground conductor is formed on the dielectric substrate and connected to the second connection terminal at one end of the ground conductor and has at the other end a terminal connected to the ground conductor on the printed wiring board.
A portion of the first half between the first open end and the first connection terminal is asymmetric in form to a portion of the second half between the second open end and the second connection terminal. Alternatively, the power supply conductor is asymmetric in form to the ground conductor on the dielectric substrate. Thereby, the total impedance of the power supply conductor and the portion of the first half between the first open end of the first half and the terminal, at the other end of the power supply conductor, connected to the high frequency signal lead and the impedance of the high frequency signal source substantially match, in total impedance, the ground conductor and the portion of the second half between the second open end of the second half and the terminal, at the other end of the ground conductor, connected to the ground conductor on the printed wiring board.
The printed wiring board of the telecommunication device according to this invention preferably has the ground-free area of the board between the ground area of the board and a side edge of the board, and the antenna element is preferably mounted on the ground-free area of the board so that the dielectric substrate surface having the radiation electrode is adjacent to the side edge of the board and a dielectric substrate surface other than the dielectric substrate surface having the radiation electrode faces the ground-free area of the board.
In the telecommunication device according to this invention, since the radiation electrode of the antenna element is spaced apart from the ground conductor on the printed wiring board, the effect of the grounding can be eliminated.
The antenna element of the telecommunication device according to this invention preferably further comprises ground electrodes, formed adjacent to the first and the second open ends on the dielectric substrate, connected to the ground conductor on the printed wiring board, each of the ground electrodes connecting capacitively to the first and the second halves at the first and the second open ends, respectively. The radiation electrode is preferably in a meandering form.
The electric conductor forming the radiation electrode may be discontinuous between the first connection terminal and the second connection terminal and divided into the first and the second halves. Alternatively, the electric conductor forming the radiation electrode may be continuous from the first half to the second half and have one of the first and the second connection terminals around the center of the radiation electrode. Each of the first and the second halves may be a quarter-wave antenna.
In the telecommunication device according to this invention, the electric conductor width of each of the first and the second halves of the radiation electrode may be narrowing from the center toward each of the open ends and the distance between the electric conductors of each of the first and the second halves may be increasing from the center toward each of the open ends.